Coral reefs are the most diverse and complex marine ecosystem and comprise the largest biological structure on the earth. For most people, they are a place of vibrant colors, high biodiversity and full of life. Recently, however, coral reefs have been facing increasing hazards and threats and many coral habitats worldwide have been declining rapidly.
Coral mortality, in the form of coral bleaching, is one of the worst problems for ocean biodiversity and climate. The ever-increasing level of carbon dioxide in the atmosphere causes rising temperatures, which in turn causes ocean acidification, impacting both fishes and coral reefs across the globe. As the human population of the Earth continues to grow and the effects of climate change become more prevalent, it would be expected to witness even worse conditions for coral reefs and marine life. This would in turn come back to bite humans, as disappearance of corals would also have impact in fishing and tourism activities. Declining coral diversity and fish population, therefore, would have an effect on all living things and would be a recipe for disaster.
The UN's 13th goal of Sustainable Development of below 2 degrees increase in temperature in the next 50 years will be impossible to withhold unless something drastic is done in the next three years.
This article will showcase the problem at hand and the correlations between climate change, with a perspective on tourism and fishing to understand the implications of coral reefs decline and what is to be expected in the coming years.
What is coral bleaching?¶
Most reef-forming corals contain symbiotic microscopic algae (zooxanthellae), which under usual conditions provide up to 90% of the energy requirement of corals. Certain environmental conditions, however, can provoke stress to the corals, and their stress response results in these algae being expelled from the coral host. Zooxanthellae contain colorful pigments, and their departure reveals the coral's white underlying calcium carbonate skeleton through the translucent tissue - the coral appears “bleached”. The images below are of a normal coral (left) and a bleached coral (right), showing the drastic change in appearance caused by coral stress.
Environmental stressors including low salinity, unusually cold temperature and increased exposure to light can result in localized coral bleaching. However, mass coral bleaching events have been linked to warm oceanic temperature anomalies, which occur on the scale of hundreds to thousands of kilometers, when ambient water temperatures exceed the coral's tolerance level. Such bleaching has an impact on the animal habitats as the symbiotic system is made vulnerable and in some cases destroyed.
Data used in our research¶
Datasets have been gathered from the NOAA, National Oceanic and Atmosphere Administration, which runs several services useful for our investigation. One of them -Coral Reef Watch- provides information on sea surface temperature. Comparison of these temperatures with long-term monthly climatology enables emission of bleaching alerts for coral reefs at their locations. Another dataset -Simple Ocean Data Assimilation (SODA)- aims to reconstruct the physical and biogeochemical history of the ocean, and has data of ph-level, co2 and other factors on a global scale. From these datasets we have analysed the years from 1986 all the way up to 2020, as those are the years were the data is found in complete form.
Additional datasets, to study the socioeconomic impact of the topic, have been gathered from other sources. The most voluminous ones was the Global Fisheries Landing dataset, which is consolidated from several sources and measures the annual tonnage of fishing in several locations up to the year 2015. A "socioeconomic dependence" dataset shows a snapshot from 2011 of consolidated data measuring the level of dependence and vulnerability of countries and territories which have coral reefs. One of the latest sections in this research will focus on the Great Barrier Reef in Australia, so tourism data will be used to measure the socioeconomic impact for that region.
This article is mostly interactive, as it lets you explore the general trends and correlations, while zooming into specific coral reef locations to showcase the problem at hand. In the map below, you can see the stations where temperature readings have been gathered from. Feel free to explore the map by zooming in and taking a closer look.
To start with, an overview of the trends of climate measurements and bleaching alerts from 1986 to 2020 has been constructed.
The figure below shows the amount of yearly bleaching alerts through the years, according a scale defined by Coral Reef Watch. The levels are as follows:
- Alert level 0: no thermal stress to the corals
- Alert level 1: bleaching watch - the temperature trend attracts attention
- Alert level 2: bleaching warning - stress will begin if temperature increases
- Alert level 3: significant bleaching expected within weeks
- Alert level 4: severe, widespread bleaching and coral mortality expected
It is known that major bleaching events occurred in 1998 and 2017, where temperatures also reached an abnormal level. In fact 1998 was a temperature record year in the 20th century. Sea surface temperature is the most accurate measure currently used to classify bleaching alerts, so it is not a surprise that global increases in temperature will result in more frequent and more severe bleaching alerts.
In the figure above, bleaching alert level 0 has been omitted, displaying only levels 1 through 4.
The global general trend of bleaching alerts shows that, in the period of time examined, alerts from higher levels become more frequent, meaning severe bleaching is becoming more and more widespread with the passage of time. Not only level 4 alerts are significantly higher, but also levels 3 and 2.
The following figure shows the trends of temperature and CO2, which follows the often described course of climate change. It is also shown how alkalinity and pH-level develops over time on a global scale. In the figure you can explore both the yearly and monthly trends by clicking on the tabs.
The emission of CO2 is already attributed to climate change, producing a rise in temperature and thereby a rise in sea levels. Still, another grueling aspect of higher concentration of carbon dioxide in the atmosphere is the release of more free hydrogen ions into the water, increasing its acidity. This contributes even more to coral bleaching and exacerbate the crisis affecting marine life.
Alkalinity works as a buffer for this acidification process, for the production of bicarbonate and the calcification of corals.
Looking at the yearly average of these measurements, it is clear that temperature, CO2 and pH show some trends. Especially CO2 and pH have very similar curves and seem to be negatively correlated. This is not so clear for alkalinity, which seems to be a little bit lower in recent years.
The seasonal trends are also shown in the figure within the monthly tab; it is noticeable that there is a higher concentration of CO2 in the atmosphere when it is summer in the southern hemisphere of the world. This could be attributed to fact that there is less landmass resulting in increased amount of data from these regions, which in turn skewes the monthly averages. Another explanation could be that the areas with more landmass and thus more trees could to a higher extent convert CO2 to oxygen in the spring when most trees grow their leafs. There is also peaks in temperature in the summer months of both hemispheres.
The pH level and CO2 concentration peak oppositely; months with higher values of CO2 are months where the pH level is lower, i.e. more acidic. The reason for this near-perfect correlation is that CO2 concentration measured is used to calculate the pH-levels.
The most noticeable feature is the very steep drop in alkalinity in the months July, September and August, perhaps attributed to the lower CO2 concentration in the atmosphere in these months. In this case, there is a much clearer trend than was seen in the yearly trends and visualizes the more seasonal dependency of these variables.
The carbon cycle is a very delicate system and it is difficult to say how everything effects coral bleaching and health in general, although there seems to be a relationship between the emission of fossil fuels and the increase in temperature and acidification.
A look at some bleaching alert heatmaps¶
Take a look at the following heatmaps, where the years of major bleaching alerts have been included; namely 1998 and 2017. The heatmaps depict the amount of level 4 bleaching alerts throughout that year. As a comparison, the year 2001 is included to show the amount of level 4 bleaching in a "normal" year.
It is seen in these heatmaps that coral reefs tend to be located around the equator, meaning higher temperatures are to be expected. The amount of alerts are also seen to be much higher in 2017: there seems to be much more frequent bleaching events all around the globe, with the exception of the Middle East. The reefs off the coast of Australia and Southeast Asia seems to be severely affected by higher temperatures and in turn increased coral bleaching. As stated before, level 4 alerts are classified as severe bleaching, so it would be expected that the coral reefs are at a high risk of mortality in these areas. For the year 2001, on the other hand, we see a lower amount of level 4 bleaching compared to 1998 and 2017. This does not necessarily mean that bleaching is not happening, rather that it is not as widespread and serious.
So far we have gotten a sense of the close relationship between temperature, acidity and coral bleaching, but to get a better idea of the relationship between bleaching and climate change, we will proceed with a more in-depth examination of the correlations with these and other variables.
How are the climate variables connected? To answer this question, one must first look at the correlation plots to analyse how the average temperature, CO2, pH and total alkalinity (talk) are linked. The correlation plots below show how some of the variables are positively or negatively connected.
It is clearly seen in the figure above that temperature, CO2 and pH are correlated. The strongest correlation is seen between pH and CO2, where an increase in CO2 causes a decrease in pH, hence a strong negative correlation. The CO2 from the atmosphere is dissolved in seawater, thereby decreasing the pH. The increase in atmospheric CO2 over time will therefore be linked to a corresponding decrease in pH. For the dataset used to contruct these visualizations, the pH variable is actually calculated directly from the measured CO2. The reason the two variables are not completely correlated, might be due to rounding of the calculated pH. The average temperature and CO2 are also correlated, showing increasing temperature relating to increasing CO2.
With global warming, it seems that the increasing temperature and CO2 will almost certainly decrease the pH levels of the water. The negative effects of the ocean acidification is not only limited to the corals, but affects these directly by removing carbonate, which is a calcifying agent needed to build and maintain the skeleton of the corals. These effects, along with the temperature increase, compound to diminish the chances coral reefs have of surviving global warming.
The most interesting correlation here is the average temperature and average CO2 as these are from measured values. There seems to be a strong correlation between these as would be expected from a climate change perspective.
The rest of the correlations reveal what has already been discussed; namely that pH is calculated from CO2, which is why the correlation is almost perfect. This is also the reason why the correlation in the Figures CO2 vs. Alkalinity and pH vs. Alkalinity are symmetric.
It would seem that some of these visualisations are not useful, but the information gained is that it can be difficult to find causality between variables. It is not trivial that pH is not a measured value, but finding this can be important for latter conclusions. Therefore, exhaustive examination is necessary for a clear analysis in order to better understand the dynamics of the oceans and coral reefs.
From the previous figures it was seen that the average yearly temperature increases over time, but is this really the case everywhere on the globe?
To help better understand the change in temperature over time the figure below depicts the temperature as a function of latitude, with latitude ranging from 90 at the North Pole to -90 at the South Pole, making 0 the location of the Equator. In the first plot the yearly maximum, mean and minimum are calculated at all measured latitudes, and show the range of temperatures at all latitudes as well.
In order to depict the general change in temperature over the years the temperature and latitude data of each selected range of years was fitted to a second order polynomial. The difference in temperature are small relative to the normal range of temperature, meaning the difference between the ranges of years might be of more interest. The difference in temperature is calculated as the fitted curve for the range of years minus the fitted curve for the period from 1982 to 1990.
The yearly max, mean and min temperature show that the general temperature range is around 10 degrees at most latitudes. As expected higher temperatures are seen around the equator, lowering towards the North and South Poles. A quite small range of temperature can be seen at latitudes of 0 to 10, where it only changes around 7 degrees. The water temperature in these areas can therefore be expected to be both higher and more constant than other regions of the globe.
The temperature differences for the fitted curves show an overall steady increase in temperature over time, but also that the overall increase in temperature is not equal across the globe. The increase in temperature is much lower towards the poles and is highest around the Equator and further north to latitudes of around 40.
The subject of our focus later, the Great Barrier Reef, has a latitude of around -16.4, meaning it is well within the range of latitudes that have seen a yearly increase in temperature of roughly 0.2 degrees celsius between the last two decades. If nothing is done to prevent it, the pattern would suggest that these areas would see further increase in temperature in the next decades.
In order to investigate the connection between temperature, latitude and bleaching alerts further, the figure below can be examined. Here the first graph shows the max, mean and min yearly temperature as a function of latitude as seen previously. On this first graph all bleaching alerts of levels 1-4 are plotted as a function of the corresponding temperature and latitude of the individual bleaching alerts. The plot is interactive such that individual bleaching alert levels can be hidden.